Premature infants show patterns of respiration characterized by brief pauses (periodic breathing) thought to be of no clinical consequence, and occasionally more prolonged pauses with bradycardia (apneic spells) which are associated with a poor prognosis. The respiratory patterns tend to become more regular with increased postnatal age. The proposed studies are undertaken to learn more about the maturation of respiratory control mechanisms in human infants of different postnatal ages. The plan is to apply new and non-invasive techniques to assess the output of the respiratory center, the contribution of pulmonary stretch receptor input, and the responsiveness of chemoreceptors. The pressure generated by the inspiratory muscles following occlusion of the airway for a single breath, an end point relatively independent of body size and lung mechanics, will be used to measure the output of the respiratory center in response to CO2 at known O2 tensions. Vagal inspiratory time at different lung volumes. Infants with and without apneic spells will be studied on several occasions to see if their responses to chemical and neural input to the respiratory center differ.